The present invention relates to a magnetic resonance imaging apparatus having a vibration damping means for suppressing a vibration of a gradient coil.
The gradient coils of a magnetic resonance imaging apparatus are usually constructed with plural pairs of saddle shaped coils which are respectively disposed in X and Y directions in a sectional plane perpendicular to a direction of a static magnetic field in a measuring space in which a body to be examined is to be placed, and are generally mounted on a cylindrical bobbin made of epoxy resin having a large elastic modulus via a rubber damper. The gradient coil generates a gradient magnetic field so as to obtain a three dimensional sectional image of the body to be examined.
When a pulse current flows in the gradient coil in order to obtain an image signal, an electromagnetic force is generated on the gradient coil in a direction crossing the static magnetic field and acoustic noise having frequencies from 250 to 8000 HZ is generated by the bobbin which is vibrated by the pulse current flowing in the gradient coil.
In order to suppress the acoustic noise generated by the vibration of the gradient coil in a conventional magnetic resonance imaging apparatus, there is provided a rubber damper between the gradient coil and the bobbin which supports the gradient coil in order to decrease the transmission of the noise from the gradient coil to the bobbin. Examples of such a device are disclosed in European Patent Publication No. 0 350 640 and International Publication No. WO 86/07459.
To a certain extent, such a rubber damper can suppress the generation of the noise which is mainly generated at the bobbin for supporting the gradient coil or at a cryostat device for generating the static magnetic field to which is transmitted the vibration from the gradient coils.
But, since the rubber damper is soft in order to act as a cushion for absorbing the noise and thereby permits the gradient coil to move, this causes a serious problem in as that the electromagnetic force caused by the pulse current deforms the gradient coil and makes it impossible to obtain an accurate image signal.
Furthermore, such a rubber damper cannot decrease the acoustic noise generated by the bobbin to a sufficiently low level because the static magnetic field has ben greatly strengthened recently and the electromagnetic force generated in the gradient coil makes the bobbin vibrate violently.